The present invention relates to an optical multi-beam scanning device which scans a plurality of light beams and can be utilized for a multi-drum type color printer, a multi-drum type color copying machine, a high-speed laser printer, a digital copying machine, and the like and an image forming apparatus in which the optical multi-beam scanning device is utilized.
A plurality of image forming units corresponding to color-separated color components and an optical scanning device (laser exposure device) which provides pieces of image data corresponding to the color components, namely the plurality of laser beams to the image forming units are used in the image forming apparatus such as the multi-drum type color printer and the multi-drum type color copying machine.
Usually the optical scanning device includes a semiconductor laser element, a first lens group, an optical deflection device, and a second lens group. The semiconductor laser element is of a light source. The first lens group focuses a beam diameter of the laser beam outgoing from the semiconductor laser element into a predetermined size. The optical deflection device continuously reflects the laser beam focused by the first lens group toward a direction orthogonal to a direction in which a recording medium is conveyed. The second lens group focuses the laser beam deflected by the optical deflection device to a predetermined position of the recording medium. Usually, the direction in which the laser beam is deflected by the optical deflection device is shown as a main scanning direction, and the direction in which the recording medium is conveyed, namely the direction orthogonal to the main scanning direction is shown as a sub-scanning direction.
When the optical scanning device includes the plurality of semiconductor laser elements to output the plurality of laser beams, the laser beams are combined with one another by an optical element to form a close-in beam, and the close-in beam is causes to contribute to formation of a latent image. The optical multi-beam scanning device described in Japanese Patent Application Laid-Open (JP-A) No. 11-218699 (cited document 1) can be cited as an example of the method of combining the plurality of light beams to form the close-in beam.
The optical multi-beam scanning device described in JP-A No. 11-218699 is characterized in that a luminous flux is included in a deflection plane after the luminous flux passes through a polarization beam splitter and the deflection plane is declined from a plane including two luminous fluxes immediately before the two luminous fluxes are incident to the polarization beam splitter. As shown in FIGS. 2A and 2B, the optical multi-beam scanning device includes at least two light-emitting portion 301, a coupling lens 302, an aperture 303, a polarization beam splitter 305, a deflector 308, and an image-formation optical element 309.
The aperture 303 is provided between the coupling lens 302 and the polarization beam splitter 305. An inner hole 303a of the aperture 303 has a shape shown in FIG. 3. Specifically the inner hole 303a of the aperture 303 is formed so as to have the circular shape smaller than a portion where two light beams overlap each other, when beam centers of the two light beams from the light-emitting portions 301 overlap in the center of the inner hole 303a. Namely, the inner hole 303a of the aperture 303 is set so as to be smaller than a range in which the light beams having substantially oval shapes are combined with each other. That is, the inner hole 303a of the aperture 303 is set so as to be smaller than a range in which the light beams having substantially oval shapes defined by more than 1/e2 of peak intensity are overlapped with each other.
However, there are the following problems in the conventional optical multi-beam scanning device.
Since the inner hole 303a of the aperture 303 is formed so as to have the circular shape smaller than the portion where the two light beams from the light-emitting portions 301 overlap each other, more than half of a light quantity from each light-emitting portion 301 is not utilized. Therefore, optical efficiency is decreased and the light from each light-emitting portion 301 cannot effectively be used.
Since a large portion of the incident light beam is rejected by the aperture 303, in consideration of the rejected portion of the light beam, it is necessary to use the light-emitting portion 301 having the large light quantity.
Since radiation angle characteristics depend on the light beam, sometimes the main scanning beam diameter differs from the sub-scanning beam diameter on the image surface. An intensity distribution on the image surface depends on the light beam, and sometimes a size of the line is changed when a thin line is formed only by one beam.
Further, there is also means for combining the light beams by polarizing the light beams with a wave plate. However, in this case, cost is increased because it is necessary that at least two wave plates are inserted into an optical path.